In a wide variety of manufacturing and steel processing applications, it is often desirable or necessary to join together sheets or strips of metallic materials such as steel or the like. Such sheets to be joined may be of the same, similar, or different materials. As in any manufacturing operation, cycle time is an important factor in determining the relative cost and applicability of particular manufacturing procedures.
Laser welding of sheets or strips of metallic materials is a relatively highly competitive industry, wherein high volume and quality are absolutely critical to the success of a particular apparatus and/or method. To achieve these goals, high part cycle rates must be achieved. Inherently, each cycle is made up of a combination of sheet handling steps, sheet movement and fixturing, and welding operations. Sheet handling operations include supplying sheets to a welding device, preliminarily locating the sheets on the welding device, and removing a welded finished product therefrom. Preliminarily aligned sheets must thereafter be moved into abutted registration along a common seam line for welding, and fixtured in place to enable accurate welding. Maximum welding speeds are limited by power requirements and quality considerations, and the finished welded product must be removed from the welding area before subsequent sheets can be brought in for fixturing.
One attempt to increase the efficiency of laser welding operations is shown in U.S. Pat. No. 4,877,939, which issued to W. Duley, et al. Particularly, Duley et al. contemplated the pretreatment of the metallic materials with radiation at a shorter wavelength, such as ultraviolet radiation from an excimer laser, to reduce the reflectivity of the material to radiation in the infrared wavelength range. The shorter wavelength radiation partially oxidized the surface of the sheet material to increase its absorptivity to infrared radiation, such as that from a YAG or CO.sub.2 laser used for cutting the material. Such pretreatment allegedly increased the cutting rates achievable with the infrared laser devices, and achieved quality equivalent to that obtained with conventional mechanical cutting apparatus.
U.S. Pat. No. 4,691,093, which issued to C. Banas, et al., discloses a laser welding device using multiple focal spots to overcome problems such as poor fit-up of mating surfaces of items to be welded, and in other applications where a broader bead profile is required. By adjustment of the twin spot focusing optics, the beam spots can be separated longitudinally along the seam to increase the meltpool link, or greater separation of the beams can allow one beam to effectively preheat the material prior to application of the subsequent laser spot. In any case, however, the speed and efficiency of the weld is limited by the power of the laser supply and the movement of the twin beams along the length of weld seam.
U.S. Pat. No. 4,857,697, which issued to M. Melville, contemplates a continuous seam welding apparatus which can include a plurality of laser beams directed onto the seam of sheet materials to be welded. Pulsed applications of laser energy are overlapped along the seam weld following cooling and stabilization of adjacent weld spots. In this way, each pulse has time to stabilize before being partially restruck by a subsequent pulse, and the weld seam is created by these alternately applied overlapping spots along the length of the butting seam between the sheet material pieces.
Another attempt to increase the efficiency of laser welding techniques is set forth in U.S. Pat. No. 4,330,699, which issued to M. Farrow. Here, the laser is modulated at an ultrasonic frequency to induce acoustic waves into the melt of a weld joint. Farrow teaches that it is best to apply the "sound laser" at lower power levels and following the application of a high powered welding laser. Both of these lasers, however, must travel over the entire length of the seam for proper application.
In applications requiring relatively long welds along a common seam line, it has also been found that problems arise in maintaining the proper gap width along the seam line to permit high quality connection. For example, in welding procedures which require joining of a seam line longer than about 30 inches (about 750 mm or more), thermal effects often cause the gap to tend to separate as the weld bead is applied at a first end, causing welding device alignment problems and compromising the efficiency and quality of the resulting weld. As used herein, the term "relatively long" will be understood to connote a seam line or weld which is sufficiently long to allow thermal effects to interfere with the maintenance of proper gap width and efficiency of the welding process. While the threshhold length may vary depending upon a number of variables including materials involved, gap width, sheet thickness, welding conditions, quality of edge cuts along the seam, and the like, thermal problems are almost always encountered when the seam line length exceeds about 750 mm.
Consequently, while many attempts have been implemented to address the continuing need to optimize laser welding speed and quality, each were limited by the length of the weld, power requirements for the laser devices applied, and effective welding speed along the length of the seam. While improvements to vision systems for monitoring the seam gap width and weld bead quality have been made, and methods for automatically aligning and fixturing sheets to be butt welded are now available, an apparatus and method for optimizing the speed of high quality laser welding to provide fast part cycle times with a relatively low capital cost has not been achieved heretofore.